1. Field of the Invention
The present invention relates to a signal light monitor and an optical amplifier using the same, which are used in an optical communication system.
2. Description of the Related Art
In an optical communication, as one means for increasing transmission capacity, a wavelength multiplexed light transmission system for transmitting a plurality of optical signals of different wavelengths by one optical fiber has been used. With an increase in demand for long-distance transmission, the use of a system having an optical amplifier interpolated in an optical fiber transmission line has become general.
Among optical amplifiers, an optical fiber amplifier using an optical fiber doped with rare earth element in its core as a gain medium and a semiconductor amplifier using stimulated emission phenomena inside a semiconductor are known. As a commercially available optical amplifier at present, an optical fiber amplifier is typically used.
In an optical communication system, in order to facilitate system maintenance, there has been an increase in demand for a function for monitoring a power of an optical light propagated on an optical fiber transmission line, its wavelength, its signal-to-noise ratio, fluctuation in a signal light wavelength, and so on, in a connection point between a terminal station device or an optical amplifier and the optical fiber transmission line. In the light amplifier, a level for sending out a light to the optical fiber transmission line is maintained constant and a transmission characteristic is made stable. Accordingly, its signal light output must be maintained constant. Also, a function for monitoring a signal light must be provided for controlling the operation of the optical amplifier.
Conventionally, as such an optical signal monitor, an optical signal power monitor installed inside an optical fiber amplifier has typically been used. In this conventional monitor, an optical branching device is disposed in the light output section of the optical amplifier. A part of a light output is branched by the optical branching device and received by an optical receiver. A pumping light output power of a pumping light source is controlled so that the level of this received light becomes constant. In this way, a light output of the optical fiber amplifier is maintained constant. However, if a signal light monitor for system maintenance employs the above-described system which is constructed in the manner that the optical branching device is disposed in the optical fiber transmission line and a branched light is received by the optical receiver, it is full light power including noise light that can be monitored. Consequently, even if this conventional system is used for the signal light monitor for system maintenance, a signal to noise ratio of a signal light, signal light power of each signal light wavelength of a wavelength multiplexed optical signal or fluctuation in a signal light wavelength cannot be monitored.
Furthermore, if the foregoing system were used as a monitor for controlling the optical amplifier, the following problem is involved in the conventional system. Generally, the optical amplifier outputs not only an amplified signal light and but also an amplified spontaneous emission (ASE) from a gain medium.
Consequently, it is impossible to monitor only signal light power in reality. A ratio of ASE light power in the entire output light power is changed according to an input light level of the optical amplifier. Thus, even if an output light level monitored by the optical receiver is controlled to be constant, signal light output power cannot be maintained constant.